Aircraft jet engines is one of the fields where high strength, high resistance to oxidation and good hot workability are required to be displayed in a balanced way. In such applications, two types of Ti alloy materials have been used: .alpha.+.beta. type Ti alloy materials typified by the composition of Ti-6% Al-4% V, and semi-.alpha. type Ti alloy materials which have the composition of Ti-8% Al-1% V-1% Mo with the greater part of the structure being composed of the .alpha.-phase. The hot workability of the second type of Ti alloy material is not as good as the first type. Neither .alpha.-type nor .alpha.-type Ti alloy materials have been employed in parts of jet engines because the .alpha.-type Ti alloy materials are poor in strength and hot workability, while the .beta.-type Ti alloy materials have low resistance to oxidation.
The Ti-6% Al-4% V and Ti-8% Al-1% V-1% Mo alloy compositions are conventionally manufactured by the following steps: hot working at temperatures not lower than 850.degree. C. (.gtoreq.900.degree. C. for the first composition and .gtoreq.950.degree. C. for the second composition); annealing; solid solution treatment at temperatures not lower than 950.degree. C; and age-hardening at temperatures within the range of 500.degree.-600.degree. C. The age-hardening step is conducted only for the manufacture of the first type of Ti alloy materials, and is not performed in the production of the second type of Ti alloy material since the age hardenability is very small.
As mentioned above, the manufacture of the conventional .alpha.+.beta. type Ti alloy materials and semi-.alpha. type Ti alloy materials involves a hot-working step which is performed at temperatures not lower than 850.degree. C. Therefore, if one wants to obtain a forged product by isothermal forging which is close to the shape and dimensions of the final product, it is necessary to employ an expensive mold that has high heat resistance and which has an intricate and smooth inner surface corresponding to the shape of the final product.
Elevated temperatures are required not only in the hot working step but also in the step of solid solution treatment of the conventional .alpha.+.beta. type and semi-.alpha. type Ti alloy materials, and this impairs the thermal economy of the overall process while causing the disadvantage of scale formation.
Under the circumstances described above, the present inventors made concerted efforts to develop a Ti alloy material that can be hot-worked and subjected to solid solution treatment at temperatures lower than those required in the conventional techniques and which can additionally be age-hardened to attain high strength. As a result, the inventors have found the following: a Ti alloy which contains 2-5% Al, 5-12% V and 0.5-8% Mo (the percents being by weight) and which satisfies the relation: 14%.ltoreq.1.5.times.(V content)+(Mo content).ltoreq.21%, with the balance being Ti and incidental impurities, exhibits the .alpha.+.beta. structure at fairly low temperatures (e.g. 700.degree. C.) and the volume ratio of the .alpha.-phase to .beta.-phase close to 1:1; the Ti alloy can be readily hot-worked at temperatures lower than those which are conventionally required; in addition, the alloy can be subjected to solid solution treatment at temperatures lower than those which have heretofore been required; furthermore, in spite of its composition, which is based on the Ti-Al-V-Mo system, this alloy can be age-hardened unlike the conventional Ti-8% Al-1% V-1% Mo alloy; and the strength of the age-hardened alloy is comparable to or greater than that of the conventional age-hardened Ti-6% Al-4% V alloy.